Rheostat



I 1,507,895 Sept. 9 1924. E A ROMS RHEOSTAT F116 June 10 1922 2Sheets-Sheet 1 j A TTORNE Y5 Sept. 9, 1924. 1,507,895

. F. A. ROJAS RHEOSTAT Filed June 10 1922 2 Sheets-Sheet 2 mvszvroza BYW g J A TTORNE Y5 Patented Sept. 9, 1924.

UNITED STATES FLOBIGEL A. ROJAS, OF NEW YORK, H. Y.

armosrur.

Application fled June 10, 1922. Serial 30. 587,418.

To all whom it may concern:

Be it'known that I, FLORIGEL A. Home, a citizen of the Republic of SantoDomingo, residing at New York, in the county of 5 Bronx, State of NewYork, have invented certain new and useful Improvements in Rheostats;and I do hereby declare the following to be a full, clear, and exactdescription of the invention, such as will enable others skilled in theart to which it appermaterial consists of a structureless mass ofelectrically non-conductive material, commonly asbestos wool,impregnated with conducting powders, such, for example, as corn ductivesilicon and graphite. This material is confined betweenelectrode plates,one of which is movable, and its resistance varied by varying thepressure exerted u n it by the plates. When the material is 0 relativelygreat lateral dimensions and slight thick- 0 ness, variations in itsresistance can he satisfactorily effected in this simple manner. When,however, it is necessary to design the resistor of less width, andgreater thickness, either because of practical space considerations, orbecause the difference of potentiol between the electrode plates may beof such a magnitude that a thin layer of resistance material would notpossess sutiicient dielectric strength to prevent volt punctures andshort circuiting of the instrument, it has been discovered that thematerial adjacent the movable plate is subjected to a much greatercompression and expansion than that adjacent the stationary plate, dueto the frictional resistance of the container walls, the internalfrictional resistance of the material itself and sometimes to gravity.In other words, where the thickness of the resistance material isconsiderable, it does not act as a true homogenously elastic body, andmost of the expansion and compression is eficcted in that part of thematerial adjacent the surface where the force is applied, that is,adjacent the moving electrode. This is, of course, objectionable. Wherethe resistance material is secured to the movable plate so that positiveexpansion as well as compression is obtained, ractically the entirepotential drop takes lluce in the expanded zone, which means t edevelopment of a high temperature at this point with consequent dangerof burning out. Even where the natural resistency of the material isrelied upon for expansion, the effective variable zone of the resistoris that immediately adjacent the movable plate, so that the range andefiiciency of the instrument are greatly curtailed. Furthermore theresistance material when employed in substantial thicknesses isstructurally weak 'and likely to separate during use with the formationof air gaps. It is an object of the present invention to provide meansfor overcoming these objections.

This object is attained by providing the resistance material atintervals along its thickness with a number of intermediate pressuredistributing elements or plates which divide the resistance materialinto zones. They are designed to move in concert with the movableelectrode but at progressively varying rates, the relative movement ofall the plates being such that the zone of resistance material betweenany pair of them is compressed substantially the same amount as thatbetween any other pair. The greater the number of these intermediateplates, the closer the compression of the material approaches that of atrue elastic body and the stronger the material structurally, but by theuse of four or five in an average instrument, practically satisfactoryresults can be had. In order to obtain this pantographic action a numberof mechanical movements might conceivably be employed. In my copendingapplication, Serial No. 546,149, the use of springs between theintermediate lates has been shown. I have secured the st results,however, by the use of a screw-threaded spindle, such as shown in thedrawings forming a part of this application.

It is a further object of this invention to devise an improvedtheremostat for automatically compensating for variations in resistancedue to the thermal ex ansion of the rheostat. The use of a metal ic discfor this urpose was disclosed by me in my U. S. atent No. 1,366,945, butthere the heating of the thermostat was primarily by conduction from theresistor. In accordance with the present invention the thermostatic discis made the stationary electrode and the current made to pass through aconsiderable part of it so that it is heated not only by conduction andradiation but in accordance with Joules law, because of its ownelectrical resistance.

Further objects of the present invention are to devise more eflicientmeans for cooling the interior of the resistor, and in general toimprove rheostats of this type.

The invention will be more clearly understood after a study of theaccompanying drawings which disclose certain referred embodiments of it.In these drawings Fig. 1 is a vertical section through a rheostatconstructed in accordance with the present invention, and Fig. 2 is asimilar view of a very similar rheostat, but showing a certain modifiedform of construction.

The rheostat shown in these drawings comprises a metallic casing 1closed at its upper end and opened at its lower. Within this casing ismounted a cylindrical porcelain shell 2 which contains the resistorproper. This resistor is supported upon a metal plate 3 secured to thecasing 1 by means of bolts or lag screws 4 and 5, which pass throughpassageways in the porcelain shell 2 and therefore serve to hold theshell in position also. The up er ends of these bolts serve as the twobin ing posts 6 and 7 of the rheostat. They are insulated from thecasing 1, and bolt 5 is also insulated from the supporting plate 3. Theinner face of this plate 3 is'provided with an annular flange 8. uponwhich is mounted a metallic disc 9. This disc performs two functions-itserves first as the stationary electrode of the rheostat and directlysupports the resistance material, and it also serves as a thermostat, aswill be described in greater detail later. Extending through andjournaled within the upper and closed end of the casing 1 is a spindle11. That part of the spindle which lies within the casing is providedwith a'se'ries of screwthreaded sections 12, 13, 14, 15 and 16. Thatpart'which lies outside of the casing terminates in a hand wheel 17 Themovable electrode 18 is mounted upon section 12 of the spindle and iselectrically connected to the bolt 5 and the binding post 7 by means oflead 10. Mounted upon the other sections 13, 14, 15 and 16 are a seriesof intermediate pressure distributing plates 19, 20, 21 and 22respectively. These plates and the movable electrode are secured to theresistance material anddivide it into a plurality of zones of equalthickness, the two outer zones contacting with the electrodes. Themovable electrode and these intermediate plates are .held againstrotation by means of'a tin or key 23 projectinginwardly from the innereeaeee face of the shell 2 and fitting withinsuitable notches 24 in theperipheral edges of the electrode and plates. It is therefore clear thatupon rotation of the spindle the movable electrode and the pressuredistributing plates will be moved longitudinally thereof and hence exerta pressure against the resistance material at a plurality of pointsalong its thickness. In the absence of the pressure distributing plates,movement of the movable electrode would be effective only against whatis shown as the upper zone of resistance material, and the lower zoneswould be neither compressed nor expanded an appreciable amount. Inorder, however, to distribute the pressure throughout the thickness ofthe material so that it will act substantially as a true elastic body,it is necessary not only that the intermediate plates exert a pressureupon the material but that the pressure exerted by any one of them mustbear a predetermined relation to that exerted by the others. In otherwords. these plates and the movable electrode must be advanced andretracted as though they themselves were elements in a true elasticbody.

This effect is produced by suitably choosing the pitches of the threadson the various threaded sections sothat when the spindle is rotated theplates and electrodes approach each other at the same rate. Forpractical purposes I have chosen the following pitches. Section 12contains five threads to the inch. section 13 six, section 14 eight,section 15 twelve and section 16 twenty-four. Thus upon one completerotation of the spindle the movable electrode 18 will move one-fifthplate 20 one-eighth of an inch, plate 21 onetwelfth of an inch and plate22 one-twenty-- fourth of an inch. The difierence between thesemovements is one-twenty-fourth of an inch, with the single exception ofthe ditl'er-. ence between the-movement of the movable electrode andplate 19. One-fifth of an inch is not exactly five-twenty-fourths, butit is close enough for practical purposes. By originally spacing theplates and the electrodes an equal distance fromeach other it is clearthat this distance will always be maintained, and the lowest zone. thatadjacent the stationary electrode, will be subjected to substantiallythe same compression and expansion as that adjacent the movableelectrode.

Thus far the description is equally applicable to Figs. 1 or 2, as bothforms of rheostatoperate on exactly the same principle. The assemblingof the parts of the resistor proper presents some difliculties and thestructures shown in the two figures illus trate two different ways inwhich this can be done. I shall first describe the assembling of theparts in connection with Fig. 1. It

Ill-3 best adhesive of this nature which I know consists of a solutionof metallic salts as described in my copending application, Serial No.367,418; With a unitary indle'and one-piece plates it would be di It toassemble the parts and secure the zones of resistance. material to thevarious contacting surfaces, as it would be necessary to rotate 7 theplates when in contact with the material. For this reason I make thespindle shown in Fig. 1 in 'a series of separable sections,

each section carrying one of the sets of" screw threads as shown, andeach section being provided with a square head 25 at one end and asquare socket 26 at the other so that they can interfit as shown. As itis also necessary that the intermediate plates be insulated from thespindle asa whole, I have interposed a layer of-insulation 27 betweeneach of the interfitting sedions. As shown, the socket of each section,:as well as the socket into which section 12 fits, is rovided with acoating of vitreous enamel. ni rder that the assembled spindle shouldnot come apart, a support isfrovided in the form of a screw 28 rovi edwith a conical tip and supported y the plate 3, passing upwardl throughan opening 29 in the disc- 9. The

I lowermost section 16 is provided with an insulating insert 31 withwhich the screw 28 makes contact.

Theresistor proper is assembled outside of the casing, the zones of theresistance material being. secured b a suitable adhesive to theirrespective .p ates, mounted upon their respective screw-threadedsections, and the parts put together without rotation of any of theplates. When assembly is complete the resistor roper is placed withinthe shell 2, the shel 2 placed within the'casing 1, the screw 28advanced to meet the end of the spindle, the proper connections made andthe rheostat is in readiness for use.

In the form shown in Fig. 2, substantiallythe same result/is obtained byprovidmg each intermediate plate with a separable hub 32 which isscrewed upon the corre Sponding screw-threaded section of the spin e.These hubs are secured to the plates by means of screws 33 and areinsulated from them. The spindle itself is also insulated from the casinThis structure also per- :mits assembly 0 the plates without rotation.Here. as the spindle 18 in one iece, the support sptiew 28 shown in Forthese two In both figures the stationary electrode is in the form of athin metallic disc 9. This disc, as stated above, also serves thefunction of a thermostat, expanding and bulgmg outwardly when hot andpermitting a slight elongation of the resistance material whichcompensates for the increase in con ductibility due to the internalexpansion of the material. In order to regulate the bul; ing action ofthe thermostat a set screw 34 is provided. In Fig. 2 this set screw islocated beneath the center of the disc. In the form shown in Fig. 1 thisposition is cocupied by the supporting screw 28 so that two set screws34 are employed, one-on either side of the et rioal center.

' I have discovere however, in the form of rheostat shown in thesefigures, the current pming from the resistor through the annular flange8 of the plate 3, that very little of the current actually passesthrough the disc 9, and therefore this disc is heated Y merely byradiation and conduction and not because of its resistance in accordancewith J oules law. This interferes with thesensitiveness of thethermostat, and in order to force a substantial uantity ofv the currentthrough the disc itsdif I have interposed an annular sheet of asbestos35 between the resistancematerial and the peripheral surface of thedisc. The greater the width of this annular sheet, the more the currentpassing through the disc itself.

In order to facilitate the cooling of the interior of the resistor Ihave drilled radial holes 36 in the intermedi rfplates, thus in-.creasing their radiatin su ace and assistin in dissipation of t egenerated heat.

he operation of the rheostat, althoug:

s, y riefly reviewed as follows: urrent enters at one of the bindingposts, say at 7. passes through the lead 10 to the movable electrode18,-through the succeeding zones of ance material and the intermediateplates.

probably'clear from the fore the stationary electrode and thermostat 9,

and the intermediate plates to move toward I d away from the stationaryelectrode. hibvement of these parts is pantogiaphic, and the instrument18 designed so t 'at the pressure exerted on each zone of resistancematerial is substantially the same as that exerted on any other, so thatthe insistence material as a whole acts under compression and expansionsubstantially like a true elastic body. By means of thisinstrumentsatisfactory use can be made of a compressible resistancerheodaat in which the resistance material-has a very substantialthickness,

terial is compressed and expands as a sub-' stantially true elasticbody.

2. In a compressible resistance rheostat,

the combination of a stationary electrode,

a movable electrode, a compressible resistance material between theseelectrodes, a plurality of plates intermediate the electrodes dividingthe resistance material into a plurality of zones. and means for movingthe movable electrode andthe intermediate plates so that each zone 'iscompressed and expanded substantially as every other zone.

3. In a compressible resistance rheostat, the combination of a pluralityof zones of compressible resistance material, and means for applyingforce to each zone so that each is compressed and expanded substantiallythe same as every other.

4. In a compressible resistance rheostat, the combination of tworelatively movable electrodes,v a compressible resistance material heldbetween them,j.a plurality of substantially equi-spaced platesintermediate the electrodes dividing the resistance material into equalzones, and means for eii'ecting a relative movement among the electrodesand .the intermediate plates to vary the pressure upon and theresistance of the resistancematerial, the electrode and plates remagsubstantially equi -spaced throughout.

5. in a compressible resistance rheostat, the combination of twoelectrodes, one stationary and the other movable, a compress ibleresistance material situated between them, a plurality of platesintermediate the electrodes dividing the resistance material into zones,means for moving the movable electrode and the intermediate platestoward and away from the stationary electrodes to vary the pressure uponand resistance of the resistance material, the movement of each beingdirectly porportional to its distance from the stationary electrode.

6. in a'compressible resistance rheostat, the combination of tworelatively movable electrodes, a resisce material confined between them,premure distributin plates in termediate the electrodes dividing theresistance material into zones, a spindle having a number of threadedsections of different pitches on which are mounted one of "theelectrodes and the intermediate plates,

means preventing rotation of the electrodes and and means ror rota' themenses spindle to move the electrodes and plates relative to one anotherand vary the pressure on the resistance material.

7. In a rheostat of the class described, a series of relatively movablepressure distributing plates dividing the resistance material intozones, a screw-threaded spindle having sections of difi erent pitches onwhich the plates are mounted,.means preventing rotation of the plates,and means for rotating the spindle to cause movement of the plateslongitudinally thereof, the relation of'the difierent pitches being suchthat the plates approach each other. at the same rate.

In a rheostat of the class described, the combination of relativelymovable electrodes, resistance material held between them, pressuredistributing means intermediate the electrodes, and pantographic meansacting upon one of the electrodes and the pressure distributing means tomove them in such a .way' that the resistance material is compressed andexpanded as a whole in substantial conformity with elastic-claws.

9. In a rheostat of the class described, a series of relatively movablepressure distributin plates secured to the. resistance inateria anddividing it into zones, a screwthreaded spindle having-sectionsofdifierent pitches on which the parts are mounted,

means preventing rotation of the plates,

means permittting assembly of the plates on the spindle withoutrotation, and means for rotating the spindle to cause movement offgeprllates longitudinally thereof.

series of relatively movable pressure distributing plates, dividing theresistance 'material into zones, a-screw-threaded spindle havingseparable threaded sections of difier-V ent pitches on which the platesare mounted; means preventing rotation of the plates, and

means for rotating the spindle to cause move-' ment of the plateslongitudinally thereof, the relation of the different pitches being suchthat the plates approach each other at the same rate.

ll. In a compressible resistance rheostat, the combination of a movableelectrode, a stationary electrode, resistance material confined betweenthem, pressure distributing plates intermediate the electrodes, a com-POSliZBSCISW-tllffifidttill spindle having separable threaded sections,the movable electrode and the intermediate plates being unted one oneach of the separable threaded sections, and a support for the spindleextend mg uprdly from the stationary electrode,

12.111 a compressible resistance rheostat, the combination of a movableelectrode, a stationary electrode, resistance material can fined betweenthe pressure distributing plates intermediate the electrode,acom= positescrew-threaded spindle ha sep arable threaded 1-: insula from each arheostat of the class described, a

ill)

other, the movable electrode and the intermediate plates being mounted,one on each of the separable threaded sections, and a support tor thespindle insulated from it extending upwardly from the stationaryelectrode 13. In a compressible resistance rheostat,

the combinatiomof a movable electrode, a metallic thermostatic discserving as a stationary electrode, resistance material between the two,and means causing the current passing through the resistance material topass through a substantial part of the thermostatic disc and heat it inaccordance with Joules law.

14. In a compressible resistance rheostat, the combination of a movableelectrode, a metallic thermostatic disc serving as a stationaryelectrode, resistance material between the two, means causing thecurrent passing through the resistance material to pass through asubstantial part of the thermostatic disc and heat it in accordance withJoules law, and means for regulating the expansion of the thermostaticdisc.

15. In a compressible resistance rheostat, the combination of a movableelectrode, a metallic thermostatic disc serving as a stationaryelectrode, resistance material between the two, means makin electricalcontact with the thermostatic disc at its periphery, an annular sheet ofinsulating material interposed between the resistance material and theperipheral surface of the thermostatic disc to cause the current to passthrough a substantial part of the thermostatic disc and heat itinaccordance with Joules law.

16. In a compressible resistance rheostat, the combination of tworelatively movable electrodes, resistance material confined betweenthem, pressure distributing plates intermediate the electrodes andhaving openings within them facilitating the cooling of the resistor.

17. A rheostat comprising the combination of a casing closed at one endand open at the other, a shell of insulating material within the casing,a stationa electrode closing the shell at the open en of the casing, amovable electrode within the shell at the closed end of the casing,resistance material held between the electrodes, and means extendingthrough the closed end of the casing for moving the movable electrode.

18. In a compressible resistance rheostat, the combination of tworelatively movable electrodes, a compressible resistance materiaL heldbetween them, a plurality of substantially equi-spaced platesintermediate the electrodes dividing the resistance material into twoequal zones, the resistance material adhering to the electrodes andplates so that it can be positively expanded as well as compressed, andmeans for effecting a relative movement among the electrodes and theintermediate plates. to vary the pressure upon and the resistance of theresistance material, the electrode and plates remaining substantiallyequi-spaced throughout.

19. In a compressible resistance rheostat, the combination of tworelatively movable electrodes, a metallic thermostatic disc serving asone of the electrodes, resistance material between the two, and meanscausing the current passing through the resistance material to passthrough a substantial part a of the thermostatic disc and heat it inaccordance with Joules law.

In testimony whereof I aflix my signature.

FLORICEL A. ROJAS.

